1. Field of the Invention
This invention pertains in general to the field of breathing systems for providing a flow of gas to a patient.
More precisely, the invention relates to determining the actual flow in such breathing systems for safety purposes.
2. Description of the Prior Art
Various breathing systems for regulating a flow of gas to a patient are known. Such breathing systems include for instance anesthesia machines, intensive care ventilators with added anesthesia capabilities, etc.
Conventional breathing systems have a flow meter at a location where it is desired to measure the gas flow. However, this adds a dead volume and may increase the flow resistance in the breathing system. Further problems with flow meters at the y-piece include difficulties in handling moisture and mucus from the patient, quick temperature changes between expiratory and inspiratory gas, and varying gas compositions, e.g. of CO2 and anesthetic agent. An alternative is to estimate a flow based on measurements at a location different from the location where it is desired to measure the gas flow.
An erroneous estimation of flow in breathing devices may involve a safety hazard potentially exposing a connected patient to situations with dire consequences, e.g. when a non-desired amount of a substance is delivered to the patient.
A source for incorrect estimation and regulation of the gas flow is the compressible internal volume of the breathing system which affects the actual gas flow, for example at a patient connection. As mentioned above, prior art apparatus often use several flow meters at critical locations in the breathing system to measure the flow.
For instance, US 2007/0089738 discloses a system for circuit compliance compensated volume control in a patient respiratory ventilation system. It is disclosed a flow regulated feedback control loop, where the patient volume is estimated. A flow meter is installed at the y-piece of the patient connection to measure the patient flow. The patient flow is used to compute a measured patient volume and the flow is continuously monitored.
EP 0723785 discloses a ventilator system comprising a connection system for connecting to a patient. A transfer function of the connection system is determined by a well known automatic control model to compensate for the influence of the connection system on the flow of gas supplied to the patient. In order to determine the transfer function several gas flows must be measured. Either a test lung is connected to the connection system, having pressure and flow meters, or the transfer function is determined without the test lung by using inspiratory flow and pressure as input signals, and expiratory flow and pressure as output signals to the automatic control model.
Having an increased number of measuring points in the breathing system do still not compensate for the actual compressive volume effect. Also, flow meters are expensive and have inherent drawbacks depending on measuring technique. There is a need for a simpler system, where the actual flow can be determined without flow meters, or where flow meters are not possible to use.
Thus, there is a need to provide alternatives or improvements avoiding the aforementioned issues. It would for instance be advantageous to be able to estimate the actual gas flow at a desired location in the breathing system taking into account the compressive volume in the breathing system. The compressive volume is particularly critical when small children and infants are connected to the breathing system, whose lung volume is comparatively small in relation to the volume of the breathing system.
It would be further advantageous to estimate the flow without the use of flow meters.
Hence, an improved or alternative breathing system allowing for determining the actual gas flow at a location such as a patient connection in a breathing system would be advantageous. This allows in particular for increased cost-effectiveness, improved reliability, versatility, and/or patients safety.